1. Field of the Invention
The present invention relates to a loop antenna for receiving data from a contactless IC card through load changes by supplying electric power and transmitting data to the contactless IC card and to a contactless IC card read/write apparatus, and more particularly, the invention relates to a loop antenna capable of attenuating the distant electric field with the attenuation of the magnetic flux in the vicinity of an electromagnetic wave restrained and to a contactless IC card read/write apparatus.
2. Description of the related art
Reader/Writer systems using contactless IC cards are generally called a contactless IC card system and have heretofore been put into practical use for physical distribution systems, traffic systems and air cargo control systems utilizing a frequency band of 13.56 MHz, for example. The reader/writer system is provided with a contactless IC card having an IC chip and an antenna coil on one sheet of plastic card and a read/write apparatus for communicating with the contactless IC card, the read/write apparatus having a loop antenna. The loop antenna is used for regularly or intermittently supplying electric power and transmitting data and for receiving data from the contactless IC card within such a range as to receive the electric power and the transmitting data
Incidentally, though high-frequency magnetic flux is utilized for communication in the reader/writer system, a high-frequency electric field other than the high-frequency magnetic flux is to be also emitted when the antenna for communication is driven. The intensity of the high-frequency electric field is regulated by the Radio Law and measures have been taken to lower the output of the antenna in order to satisfy the Law; in this case, however, there has developed a problem of decreasing the communication distance. As another measure, it has been arranged to dispose a shield plate around the antenna.
FIG. 19 is a schematic perspective view of a conventional contactless IC card read/write apparatus. Tn (JP-A-2001-326526), for example, disclosed is a shield antenna coil 33 essentially consisting of electric-field shield patterns 31 and a power-supply pattern coil 32. The electric-field shield patterns 31 are wide enough to cover the power-supply pattern coil 32. The electric-field shield patterns 31 are in the form of an open loop in order to prevent the generation of an eddy current as an obstacle to the radiation of the magnetic flux component and so disposed as to cover the power-supply pattern coil 32, the electric-field shield patterns 31 being grounded as shown in FIG. 19. With this arrangement, while the magnetic flux component required for communication is secured, it is indicated that the electric-field component caused to disturb the communicating operation of any other wireless installation is made reducible thereby.
However, though the constitution of the conventional contactless IC card read/write apparatus allows the electric field to be reduced, the attenuation of the neighboring magnetic flux required for communication tends to grow greater and the problem is that the communication distance becomes extremely shortened.